On Aug 25, 2008, at 3:00 PM, Ingvar von Schoultz wrote:
> ...we'll have to write something like this?
>
> const
> { DatabaseName,
> DisplayCount,
> ... Ten more ...
> } =
> Debugging ?
> { DatabaseName: "TestDatabase",
> DisplayCount: 5,
> ... Ten more ...
> }
> :
> { DatabaseName: "RealDatabase",
> DisplayCount: 15,
> ... Ten more ...
> };
>
> Very often conditionals aren't that simple. It'll get ugly. The
> new awkwardness does makes sense in big projects, but only there.
Another way of looking at this is to say that const should be write
once, not necessarily initialized in the declaration. Then you could
write
const DatabaseName, DisplayCount, /* ... ten more ... */;
if (Debugging) {
DatabaseName = ...;
} else {
etc.
}
This was ES4's const. At the Oslo meeting there was strong opposition
to write-once. The reasons were not clear to me (could be my fault).
Here you show a reason for write-once. The alternatives are
destructuring from a ternary (as you showed), falling back on var
(losing), or inventing some super-const binding form that escapes its
block (not in the cards).
>> let
>> blocks and let expressions do not have solid support in TC39.
>
> I have no opinion on whether let blocks and let expressions
> are useful, but you could make them very cheap by dropping the
> intricacies that make let(x=x){} refer to two different x'es:
>
> let (a = x) {...} desugars to {let a = x; ...}
If by "cheap" you mean easy to implement, there would be a savings to
compiler costs, but not at runtime.
> let (a = x) a; desugars to {let a = x; a;}
Sorry, no: the latter is not an expression, the former is (if you
remove the ; at the end).
> As a bonus everyone will rejoice at the lessened cognitive burden
> of intricacies.
But then why have the let block form at all, since let declarations
are sweet enough.
The case for let expressions remains even if you eliminate the let
binding rule, since they are expressions. You'd have to invent
generalized BCPL- or Algol-68-like expression language support, which
is also not in the cards (I mentioned the idea briefly in Oslo, more
as a joke or an aside).
> Of course Schemers will find a let with these semantics confusing.
> Clearly it needs a different name. How about... "local"? :-D
Sorry, no; please stop nagging.
/be
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